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Four days after a magnitude-8.9 earthquake rocked Japan, the shell-shocked nation faces multiple crises: a search-and-rescue effort, a humanitarian emergency and now, most urgently, the risk of full-fledged nuclear disaster.

Just after 6 a.m. Tuesday local time, as workers continued to try to pump seawater into three crippled nuclear reactors at the Fukushima Daiichi nuclear plant to cool their fuel rods, an already grim situation turned dire. Yet another explosion  the third in four days  rocked one of the three reactors that was up and running when the earthquake and tsunami hit Friday. A no-fly zone was imposed over Fukushima's 19-mile (30 km) radius, with authorities ordering 140,000 people to seal themselves indoors. (See TIME's complete coverage of the crises unfolding in Japan.)

The blast occurred at unit No. 2, which had overnight become the primary source of concern for the Tokyo Electric Power Co. (TEPCO), the plant's owner and operator. Later Tuesday morning, Chief Cabinet Secretary Yukio Edano acknowledged that "there is a very high probability a portion of the container vessel was damaged" but later said that radiation readings at the plant's front gate had returned to a level that would not cause "harm to human health." The blast also apparently led to a fire that broke out at one of the three reactors at Fukushima Daiichi that had not been running at the time of the tsunami  unit No. 4  but which does contain spent nuclear fuel. The fire burned for nearly six hours before workers managed to douse it around 1 p.m. Tuesday. But Japanese authorities said radiation levels dropped several hours after they had spiked sharply this morning. Nevertheless, after the fire was extinguished, according to the AP, a Japanese official said the pool, where used nuclear fuel is kept cool, might be boiling.

The breach of the containment vessel at unit No. 2, as well as the fire at unit No. 4, led to a spike in the radioactivity levels around the plant to dangerous levels, as a grim-faced Japanese Prime Minister Naoto Kan acknowledged in a brief nationally televised speech this morning. "The reading seems very high, and there is still a very high risk of further radioactive material coming out," Kan warned. TEPCO sent 800 of its workers home in order to avoid exposure to the increased levels of radiation, though 50 remained behind, still trying frantically to cool the fuel rods with seawater.

TEPCO's attention is now focused on the apparent breach in the containment vessel of reactor No. 2. The nuclear fuel rods that produce energy are housed in what amount to steel vaults  meant to prevent what is inside from ever getting out. Even if the fuel rods melt, partially or completely, the containment vessel is supposed to prevent the worst from happening. The question now is, If there is a breach in vessel No. 2, how serious is it? "If the primary containment [vessel] is leaking, then a core meltdown could lead to a very large release of radioactivity to the environment," says David Wright, a co-director of global security at the Union of Concerned Scientists in the U.S.
(See "Blast, Fuel-Rod Exposure Raise Fears of Japan Meltdown.")

Japanese authorities extended the exclusion zone around the plant from a radius of 12 miles (20 km) to 19 miles (30 km) and instructed people to stay indoors and shut off air-conditioners. As far south as Tokyo, atmospheric radiation levels had begun to rise (though still to levels that, for now, do not carry health risks). Vehicle and pedestrian traffic in the capital city was somewhat lighter than normal, but there was no sense of panic, except in the stock market, where fears of a complete nuclear disaster prompted panicked selling, driving the Nikkei down 11% and TEPCO's shares down 40%.

The problems at reactor unit No. 2 started over the weekend, when workers were unable to fill that unit with seawater, thanks to a malfunctioning valve. That prevented workers from releasing pressure from the reactor  which must happen before it can be filled with seawater. TEPCO then acknowledged that the fuel rods in unit No. 2 had been completely uncovered for a few hours  something "that could lead to a very serious melting of the fuel," Wright says.
(See how to stop a nuclear meltdown.)

Some nuclear scientists had already warned that the containment vessels in use at Fukushima Daiichi were at risk. A recent report by Sandia National Laboratories noted that the type of reactors at work at Fukushima Daiichi are "unusually vulnerable to containment failure in the event of a core-melt accident." The most likely failure scenario, the Union of Concerned Scientists' Edwin Lyman wrote, "involves the molten fuel burning through the reactor vessel, spilling onto the containment floor, and spreading until it contacts and breeches the steel containment-vessel wall."

It is unclear as of yet whether the apparent breach at reactor No. 2 will lead to a catastrophic release of radiation. A nuclear-industry source in Tokyo, who is included in periodic briefings on the situation at Fukushima Daiichi, said it is likely that there has been at least partial fuel-rod melting at all three of the reactors there that had been in use. The fact that there is a problem in one of the containment vessels, the executive added, "is obviously not good news."